I’m able to speak German (native speaker) and English (fluent).
Also, as a German speaker, I’d like to correct the question in the post:
Formal would be “Welche Sprachen sprechen Sie?”.
More fitting for a casual environment (such as Lemmy) would be “Welche Sprachen sprecht Ihr?” though :)
This is, because in German there are formal and informal ways of addressing people, both with their distinctive pronouns. Usually, when talking to people you don’t know personally, you’ll address them formally and then, when offered to, switch to the informal style once you know them. Online or among the younger generation it is much more common to just use the informal case though.